1. Field of the Invention
This invention relates to the detection of multipath components in wireless communication systems and, in particular, to a method and system for optimizing the multipath delay estimation quality based on interference estimates.
2. Description of the Related Art
In code division multiple access (CDMA) and wideband CDMA (WCDMA) mobile communication systems, such as the Universal Mobile Telecommunication System (UMTS), data is transmitted using a spread spectrum modulation technique wherein the data is scattered across the entire range of available frequencies. Each channel is assigned a unique spreading code which is used to spread the data in such a way that only the same code may be used to recover the data. The spreading code is called a pseudo-random noise (PN) code and is composed of a binary sequence of 1's and 0's (or 1's and −1's), called “chips,” that are distributed in a pseudo-random manner and have noise-like properties The number of chips used to spread one data bit, or chips/bit, may vary and depends, in part, on the data rate of the traffic channel and the chip rate of the system. To recover the transmitted data, the received signal must be despread with the same spreading code using the same chip rate. Furthermore, the timing of the demodulation must be synchronized, that is, the despreading code must be applied to the received signal at the correct instant in time.
Achieving the proper timing can be difficult due to multipath fading effects where the same transmitted signal travels along multiple paths to arrive at different times at the receiver unit. Referring to FIG. 1, for example, the receiver unit 100 may receive the transmitted signal from a base station 102 on a direct and unobstructed propagation path (Path 1). However, many other propagation paths (e.g., Path 2, Path 3) also exist because, in most cases, the transmit antenna of the base station 102 is not narrowly focused in any given direction. Thus, the same signal may be received again by the receiver unit 100 some time later as the signal is reflected off various objects and obstacles (e.g., a house 104, a building 106) in the surroundings before arriving at the receiver unit 100. Likewise, transmission from the receiver unit 100 to the base station 102 may also experience similar multipath fading effects.
Most CDMA based systems use RAKE receivers that are capable of identifying and tracking the various multipath signals for a given channel. Multipath signals with similar propagation distances may then be combined, depending on the time resolution of the transmission system and the instantaneous phase relationship of the multipath signals, to form a distinct multipath component. Each multipath component is assigned a despreader (RAKE finger) that has a copy of the spreading code, but which copy has been delayed in time relative to the spreading code used for the direct path component. The amount of delay time in the despreader is set to match the path delay of the corresponding multipath component. After despreading, the multipath components from the various despreaders are coherently combined to produce an estimate of the data or symbols being transmitted.
For the above arrangement to be effective, the RAKE receiver requires up-to-date knowledge of the multipath delays of the channel. This knowledge is important in order to maximize the signal-to-interference ratio of the detected multipath signal. In addition, the smaller the number of paths available at the receiver unit, the larger the probability that the detected paths may experience simultaneous deep fade. This utilization of diversity, or lack thereof, may lead to serious and often catastrophic degradation of the block error rate (BLER).
One way to identify the multipath signals is to perform a search for the paths over a whole range of possible despreading delays. The path searching can be performed by transmitting a pilot signal from the base station and applying a series of predefined despreading delays at the receiver unit. Where the predefined delays happen to match the arrival times of the multipath signals, a larger-magnitude channel estimate will result, thus indicating the existence of a multipath component. However, the processing resources and power consumption expenses of frequently executing this path searching routine is usually prohibitive.
Moreover, a path search may not always discover all available paths. Overlooking available paths by the path searcher leads to performance degradation due to various reasons. For example, a lower number of tracked paths at the receiver (utilization of diversity) leads to a higher probability that the path may experience simultaneous deep fade, causing serious and often catastrophic degradation of the block error rate (BLER). In addition, the signal-to-interference ratio (SIR) may be decreased since the paths not detected by the path searcher still act as sources of interference to the other fingers in the RAKE receiver. Further, where transmission power control is used, the increased base station transmission power towards the designated user, due to a low SIR, may increase interference to other users in the network.
A straightforward way to ensure the capture of all available paths is to run the path searcher periodically over the whole range of delays covering the maximum allowed delay spread. However, especially in the case of fast fading, the success in detecting individual multipath components depends on the instantaneous magnitude of the path at the time the searcher is run. This approach will provide good energy capture when the searcher is used fairly often, but is usually not practical because of the power consumption and signal processing resource costs mentioned above. Instead, it would be more efficient to execute the path searcher when a determination has been made that an update is needed based on some objective criteria.
Accordingly, it is desirable to be able to optimize the RAKE receiver performance while avoiding frequent path searcher re-runs. More specifically, it would be desirable to the able to detect a situation where new or additional multipath components are available in a channel, but are not presently included in the RAKE processing, and to activate the path searcher based on the occurrence of such situations.